Hydraulic brake devices equipped with an actuator that automatically regulates braking liquid pressure in wheel cylinders of a vehicle such as a two-wheeled motor vehicle or a passenger car without the intervention of an operation by the driver are known. For example, anti-lock braking systems (ABS) are coming to be used for brake control in two-wheeled motor vehicles. An ABS generally includes wheel speed sensors, an electronic control unit (ECU), and a hydraulic unit such as a hydraulic circuit. An ABS automatically controls hydraulic pressure in the wheel cylinders, without the intervention of an operation by the driver, when it detects a tendency for the wheels to lock. Further, in vehicles such as two-wheeled motor vehicles and passenger cars, electronic brake-force distribution (EBD) systems, which are systems that appropriately distribute the braking force applied to the front and rear wheels in accordance with driving conditions, are known.
The operation of a general ABS will be briefly described with reference to the hydraulic circuit shown in FIG. 1. FIG. 1 schematically shows a hydraulic circuit on the front wheel side of a two-wheeled motor vehicle. The front wheel hydraulic circuit of FIG. 1 is configured from a brake lever 101 that is operated by the driver, a master cylinder (M/C) 103 that is connected to the brake lever 101, a conduit 104 that is connected to the master cylinder 103, a charging valve (EV) 113 that is connected to the conduit 104 and is a normally open electromagnetic valve, a conduit 114 that is connected to the charging valve 113, and a wheel cylinder (W/C) 115 that is connected to the conduit 114 and operates the front wheel brake. Moreover, a pressure decreasing valve (AV) 123 that is a normally closed electromagnetic valve is connected to the conduit 114. Further, a conduit 124 is connected to the pressure decreasing valve 123, and a reservoir 125 is connected to the conduit 124. Moreover, a suction port of a hydraulic pump 119 is connected via a check valve to the conduit 124, and a discharge port of the hydraulic pump 119 is connected to the conduit 104 via a check valve. The hydraulic pump 119 is driven by a DC motor 118.
In the hydraulic circuit of FIG. 1, a hydraulic unit 100 operates when an unillustrated electronic control unit (ECU) detects a tendency for the front wheel to lock from the front wheel speed detected by an unillustrated front wheel speed sensor disposed in the front wheel at a time when the brake lever 101 is operated.
In this case, the ECU closes the charging valve 113 and opens the pressure decreasing valve 123 to allow the brake fluid in the wheel cylinder 115 to escape to the reservoir 125, whereby the ECU lowers the hydraulic pressure in the wheel cylinder 115 and eliminates the locking of the front wheel. When the locking tendency is sufficiently eliminated, the ECU closes the pressure decreasing valve 123 and opens the charging valve 113 to send the brake fluid to the wheel cylinder 115 and increase the pressure to the wheel cylinder 115. Further, the ECU appropriately closes both the charging valve 113 and the pressure decreasing valve 123 to hold the wheel cylinder pressure at a constant. In this way, in the ABS operation, the ECU performs a pressure decreasing operation, a pressure increasing operation, and a holding operation several times.